-LRB- CNN -RRB- -- American Airlines has found cracks in large pylons that hold engines onto the wings of two of its 767-300 aircraft , according to the airline and to the planes ' manufacturer .

Concerned the problem may exist on other airline fleets , Boeing says it plans to advise airlines to inspect the parts more frequently than the current recommendation of every 1,500 takeoff and landing cycles .

`` This is considered a safety issue . You 're talking about the strut that connects an engine to the wing , '' said Boeing spokesperson Peter Conte .

American Airlines spokesman Tim Wagner said the problem was first discovered on one of its 767-300s about two weeks ago during a maintenance check targeting other parts of the aircraft . The airline then found cracking on one more 767-300 after inspecting a total of 56 aircraft -- 767-300 and 767-200s -- a process American Airlines completed Monday night .

`` We found these issues when it 's best to find them , which is early on , '' said Wagner . The airline has sent the first damaged pylon to a metallurgy lab to try to determine why it cracked .

`` Until that metallurgy testing is completed no one knows what the cause is , '' said Wagner . The damaged planes were among American 's older aircraft , Wagner added .

American flies Boeing 767-300s on international routes as well as long-haul domestic routes . The 767-200s fly primarily transcontinental routes .

Airlines have previously found similar cracks at the holes where the pylon attaches to the wing , which led Boeing and the Federal Aviation Administration five years ago to recommend checks every 1,500 cycles . At that time , when the FAA issued an airworthiness directive it said , `` We are issuing this -LRB- directive -RRB- to prevent fatigue cracking in the primary strut structure and reduced structural integrity of the strut , which could result in separation of the strut and engine . ''

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American Airlines found cracks on two 767-300 aircraft

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Boeing plans to advise airlines to inspect the parts more frequently

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Cause is still unknown , testing is underway